1. Field of the Invention
The present invention relates to a sheet stacking apparatus which realizes good alignment property and stacking property regardless of a sheet stacking amount. Specifically, the present invention relates to a sheet stacking apparatus which is provided in an image forming apparatus, such as a copying machine, a printer, and a facsimile machine which forms an image on a sheet.
2. Description of the Related Art
As illustrated in FIG. 11, for example, in a conventional sheet stacking apparatus, a discharging portion is configured by sheet discharge rollers 301, counter rollers 302, and springs 304. Further, the counter roller 302 is supported by a counter roller holder 303. The counter roller holder 303 is urged by the spring 304 in a direction which pushes the counter roller 302 onto the sheet discharge roller 301. The other end of the spring 304 is supported by a sheet discharge frame (not illustrated). In FIG. 11, a sheet 14 is discharged.
As illustrated in FIG. 10, a stiffness applying rib 312 is a deforming member, and a spring 313 is an urging member for urging the stiffness applying rib 312 toward the sheet discharge roller 301 side. A movement of the stiffness applying rib 312 is controlled by the sheet discharge frame and the like so that the stiffness applying rib 312 can freely move only in a vertical direction, and so as not to move beyond a certain fixed height (a position higher than a nip of the sheet discharge roller 301 and the counter roller 302). The sheet discharge roller 301 and the counter roller 302 constitute the discharging portion which discharges sheets. The counter roller 302 has a kick-out protrusion 302a which abuts on a rear edge of a sheet to push out the sheet to be discharged (see Japanese Patent Application Laid-Open No. 08-127453).
In a conventional sheet stacking apparatus, a sheet guiding unit 312a of the stiffness applying rib 312 is set up at a higher position than a nip position 301a of the sheet discharge roller 301 and the counter roller 302, so that stiffness can be applied to the sheet to be discharged. Thus, curling of the discharged sheet in a sheet conveyance direction is decreased, so that the alignment property and the stacking property improve. More specifically, a leading edge of the discharged sheet can be prevented from hanging down and curling up on a sheet discharge tray. Further, the stiffness applying rib 312 is urged toward the sheet discharge roller 301 side by the spring 313. When a stiff sheet, such as a thick sheet of paper, passes through the discharging portion, since the stiffness applying rib 312 is lowered as far as a position where balance is maintained between the sheet stiffness and the spring 313, sheet jamming can be prevented.
As described above, the conventional technique aims to improve the alignment property and stacking property by applying stiffness to the sheet to decrease the curling in the sheet conveyance direction. However, there is a problem that the stiffness of the sheet becomes a resistance when a sheet stacking amount is close to full. More specifically, if stiffness is applied to the sheet which is discharged while rubbing an upper surface of an already stacked sheet, the resistance increases. Thus, the sheet to be discharged may be damaged or abut on the rear edge or the upper surface of the sheet already stacked on the sheet discharge tray and push that sheet out.